Fix clippy findings for Rust 1.74

[IvanIsCoding]

Some minor improvements suggested by clippy. They are currently blocking all CI actions

[mtreinish]

LGTM, thanks for fixing this. Although I thought we pinned clippy to our msrv to avoid having to do this every release.

[coveralls]

Pull Request Test Coverage Report for Build 6938826950

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• 108 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-0.7%) to 95.882%

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Files with Coverage Reduction	New Missed Lines	%
src/isomorphism/vf2.rs	1	96.1%
src/matching/mod.rs	1	98.53%
src/token_swapper.rs	1	96.0%
src/json/mod.rs	2	95.0%
src/lib.rs	2	99.45%
src/link_analysis.rs	2	95.69%
src/tree.rs	2	96.0%
src/union.rs	2	95.6%
src/dag_algo/mod.rs	3	98.78%
src/connectivity/mod.rs	4	96.23%

Totals
Change from base Build 6791568926:	-0.7%
Covered Lines:	15577
Relevant Lines:	16246

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💛 - Coveralls